1. Field of the Invention
The present invention relates to a light emitting device with a light emitting element that has a film containing an organic compound that emits fluorescent light or phosphorescent light upon application of electric field (the film is hereinafter referred to as organic compound layer), and to a method of manufacturing the light emitting device.
In the present invention, a light emitting element is an element that has an organic compound layer between a pair of electrodes and the term light emitting device includes an image display device which uses this organic light emitting element. Also, the following modules are all included in the definition of the light emitting device: a module obtained by attaching to a light emitting element a connector such as an anisotropic conductive film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package); a module in which a printed wiring board is provided at an end of the TAB tape or the TCP; and a module in which an IC (integrated circuit) is directly mounted to a light emitting element by the COG (chip on glass) method.
2. Description of the Related Art
Light emitting devices, which are characterized by their thinness and light-weight, fast response, and direct current low voltage driving, are expected to develop into next-generation flat panel displays. Among light emitting devices, ones having light emitting elements arranged to form a matrix are considered to be particularly superior to conventional liquid crystal display devices for their wide viewing angle and excellent visibility.
It is said that light emitting elements emit light through the following mechanism: a voltage is applied between a pair of electrodes that sandwich an organic compound layer, electrons injected from the cathode and holes injected from the anode are re-combined at the luminescent center of the organic compound layer to form molecular excitons, and the molecular excitons return to the base state while releasing energy to cause the light emitting element to emit light. Excitation state includes a singlet exiton and a triplet exiton, and it is considered that luminescence can be made through either excitation state.
Light emitting devices having light emitting elements arranged to form a matrix can employ passive matrix driving (simple matrix light emitting devices), active matrix driving (active matrix light emitting devices), or other driving methods. If the pixel density is large, active matrix light emitting devices in which each pixel has a switch are considered to be advantageous because they can be driven with low voltage.
In an active matrix light emitting device, a thin film transistor (hereinafter referred to as TFT) is formed on an insulating surface, an interlayer insulating film is formed over the TFT, and an anode of the light emitting element is formed to be electrically connected to the TFT through the interlayer insulating film. The material suitable for the anode is a transparent conductive material having a large work function, typically, ITO (indium tin oxide).
An organic compound layer is formed on the anode. The organic compound layer includes a hole injection layer, a hole transporting layer, a light emitting layer, a blocking layer, an electron transporting layer, an electron injection layer, etc. The organic compound layer may be a single layer that emits light, or may have a combination of the above-mentioned layers.
After forming the organic compound layer, a cathode is formed to complete the light emitting element. The laminate of the anode, cathode, and organic compound layer corresponds to the light emitting element. The material used to form the cathode is a metal having a small work function (typically a metal belonging to Group 1 or 2 in the periodic table) or an alloy containing the metal.
A first insulating layer is formed from an organic resin material to cover an end of the anode. The first insulating layer is provided to prevent short circuit between the anode and the cathode that is formed after the anode is formed.
The transparent conductive film used as the anode transmits visible light and therefore allows light emitted from the organic compound layer to pass therethrough. However, the transparent conductive film has a drawback of high resistivity compared to the resistivity of a metal. High film resistance of the anode formed of the transparent conductive film brings difficulty to injection of carriers and lowers the number of carriers that are re-combined in the light emitting element. Less recombinations in the light emitting element correspond to the light emission mechanism of the light emitting element ceasing to function. As a result, the light emitting element cannot emit light at a desired luminance.